Do you have to adjust a machine or tool to a certain distance? You might struggle doing this with a ruler every time you set up your machine.
If you have a standard distance, you can use a premeasured, off-the-shelf item to do this, such as a dowel rod.
You can also create a set of items to use for spacing.
This set of spacers was created to make it easy to change the setup from anywhere from 1/8" to 4"